The development of industry has an inseparable relationship with the treatment of industrial wastewater essentially occurring therefrom, and techniques for treating industrial wastewater are continually advanced with the development of industry.
Many attempts have been made to solve environmental pollution problems due to increased amounts of refractory COD-causing pollutants, which are not degraded in the natural environment and are difficult to decompose even using biological treatment processes such as active sludge processes. Such refractory COD-causing pollutants may include aromatic benzene ring compounds, such as chlorobenzene, nitrobenzene, decahydronaphthalene, benzene, cresol, xylene, tetrahydronaphthalene, tetrahydrofuran, toluene, phenol, ethylphenol, ethylbenzene, and pyridine, and halogenated organic compounds, such as trichloroethylene, tetrachloroethylene, perchloroethylene, and pentachlorophenol, and are contained in large amounts in various kinds of industrial wastewater, including wastewater from fiber-dyeing or paper-making processes.
In the case of nuclear power plants, ethanolamine (ETA) wastewater, which contains sulfuric acid and high-concentration organic materials generated upon operating a condensate polishing plant (CPP), is mixed with other kinds of system water, transferred to a wastewater disposal facility, subjected to physicochemical treatment including coagulation and precipitation, and then filtered.
However, COD (Chemical Oxygen Demand) and T-N (Total Nitrogen) pollutants from the ethanolamine compounds generated upon operation of CPP have not yet been effectively treated, and there is a need to develop treatment methods thereof, which may actively respond to environmental laws that are expected to become more stringent in the future.
Examples of chemicals currently useful as the pH controller in secondary system water of nuclear power plants may include ammonia, morpholine, and ethanolamine (ETA). However, ethanolamine, rather than ammonia, is used as the pH controller in not only domestic but also foreign nuclear power plants. Even when ethanolamine is used in a small amount under conditions of high temperature and high pressure, a high pH may be maintained, and thus the load of a condensate polishing plant (CPP) may be decreased, and a cation exchange resin has high sodium selectivity in an amine mode to thus minimize the introduction of sodium into a vapor generator and the corrosion of the vapor generator. The use of ethanolamine as the pH controller in place of ammonia is continuously increasing at present.
When ethanolamine is used in this way, however, COD and T-N in the effluent originating from wastewater generated upon the operation of CPP are increased. The system water, which circulates in the secondary system, is periodically regenerated to remove impurities using the ion exchange resin of CPP. As such, compounds such as ethanolamine or hydrazine and ionic materials in the system water are discharged together. The compounds, ranging in concentration from hundreds to thousands of ppm, contain large amounts of nitrogen compounds and organic materials, and are expressed as COD or T-N.
Conventional methods of lowering COD by treating the refractory pollutants include physicochemical treatment methods and biological treatment methods.
Specific examples of the physicochemical treatment methods include activated carbon adsorption, Fenton oxidation, ozone treatment, photocatalysis, UV irradiation, etc., and specific examples of the biological treatment methods include biological treatment using highly active microorganism strains, two-stage aeration, batch-activated sludge processes, anaerobic filtration, etc.
Although the biological treatment method is naturally friendly and is thus typically applied to wastewater treatment, it suffers from the discharge of sludge in a large amount, low treatment efficiency, and a long reaction time, and is also problematic because a large space is required and the costs of facilities and biological agents are high. Hence, industrial wastewater is mostly treated using a combination of biological treatment and physicochemical treatment.
Useful as the physicochemical treatment method for treating refractory COD materials, the Fenton oxidation process adopts a Fenton reaction, which is the oxidation of organic material published by H. J. H Fenton, 1894, in which divalent iron ions and hydrogen peroxide are allowed to react to produce a hydroxyl radical (.OH) having strong oxidation capacity, the method being known to be effective at oxidizing pollutants. However, since iron sulfate or the like used for the Fenton reaction impedes the Fenton reaction due to the high concentration of sulfuric acid ions, an excess of hydrogen peroxide is required, thus negating economic benefits, and furthermore, a large amount of sludge is generated due to the iron sulfate that is added. Moreover, the Fenton reaction is effective only under oxidation conditions and is very sensitive to pH, and thus, it is necessary to precisely control the pH.
The ozone treatment method is performed in a manner in which refractory materials are oxidized and removed using, as a very strong oxidizing agent, ozone, which is obtained by coupling three oxygen atoms with each other. Such ozone is prepared through electrical discharge, photochemical reaction, etc., and an electrical discharge process, which enables the highly efficient preparation of ozone in a large amount, is the most typical. However, ozone is an oxidizing agent that is preferably used under alkaline conditions because ozone decomposes under alkaline conditions to thus produce a hydroxyl radical that functions as an oxidizing agent. However, such ozone is not easily dissolved in water, and is sensitive to pH, like the Fenton reaction. It emits a unique pungent odor even at a low concentration of 0.02 ppm or less, and is known to be harmful to human bodies upon long-term exposure at a concentration of 0.02 ppm or more.
In addition to the physicochemical and biological treatment methods described above, there are devised electrochemical treatment methods in which an electrochemical principle is directly applied to the treatment of refractory pollutants to treat high-concentration wastewater.
The electrochemical treatment technique includes direct oxidation on an electrode surface and indirect oxidation using various reactive chemicals and OH radicals produced due to the water decomposition. However, in the treatment of wastewater, which contains refractory materials at high concentrations and in which electro-oxidation slowly progresses, the electrochemical treatment process is problematic because the processing time is long and excessive power is consumed, thus negating economic benefits.
Moreover, in addition to the electrochemical treatment method, chemical treatment methods using water and inorganic peroxides (sodium percarbonate, sodium perborate, sodium persulfate, ammonium persulfate, chromium persulfate, and sulfuric acid dihydrazine) have been proposed, and are effective at decreasing the amounts of refractory COD-causing pollutants but utilize expensive chemicals, most of which are imported, making it difficult to ensure economic benefits.